With the progress of practical application of optical communication systems by the development of optical fibers, development of various optical communication devices using an optical waveguide structure has been demanded. In general, characteristics required for optical waveguide materials include low light propagation loss, possession of heat resistance and humidity resistance, and controllability of the refraction index and film thickness. With respect to these requirements, silica-based optical waveguides have hitherto been chiefly investigated.
However, in construction of optical fiber networks inclusive of WDM communication, it is essential to reduce costs for fabricating various devices. Accordingly, in order to apply polymer materials that can be mass-produced and subjected to large-area processing to optical waveguide materials, organic materials inclusive of polymethyl methacrylate, a polycarbonate and polystyrene have been investigated in recent years. However, when such polymers are subjected to hybrid integration with a laser diode, a photo diode, etc., they have the defect that the range of their use is very limited, because their heat resistance in a solder reflow step is not sufficient. Of a number of polymer materials, polyimide resin-based materials have the highest heat resistance, so that they have recently attracted a great deal of attention as optical waveguide materials.
Optical circuits made of a polyamide resin have hitherto been generally formed by the following dry process. That is, a polyamic acid as a polyimide resin precursor is first dissolved in a polar solvent such as N,N-dimethylacetamide or N-methyl-2-pyrrolidone to prepare a polyamic acid varnish, which is then applied onto a substrate by spin coating or casting and heated to remove the solvent and to allow the polyamic acid to undergo ring closure for imidation, thereby forming a polyimide resin film, followed by pattern formation by reactive ion etching (RIE) using oxygen plasma etc.
However, according to the conventional dry process in which the polyimide resin film is subjected to reactive ion etching to form a pattern as described above, not only it takes a long period of time to form an optical circuit, but also the problem of reducing costs is not solved yet, because a processing region is restricted. Further, according to such a dry process, a wall surface (side surface) of the pattern formed is not flat, so that scattering loss becomes large during wave guiding of light into the optical circuit.
Consequently, as an inexpensive process for producing optical waveguides without using such a dry process, for example, a so-called release layer process is known (see, for example, Patent Document 1). However, also in this release layer process, there is fear that material characteristics, as well as the durability and mold release properties of a transfer mold, are degraded by immersing a member in an etching solution in order to remove the release layer.
In contrast, as already known, when pattern formation using a polyimide resin is conducted by a wet process using a photosensitive polyimide resin precursor composition in which a 1,4-dihydropyridine derivative is incorporated as a photosensitive agent (see, for example, Patent Documents 2 to 4), the above-mentioned problems are not encountered. However, a new problem of light loss with respect to the resulting polyimide resin must be solved. Specifically, in order to use the polyimide resin by the above-mentioned wet process as an optical waveguide material, the polyimide resin is required not to absorb a light to be guided, namely, it must be low in loss to a light, and in brief, it must have transparency.
However, in order to subject the above-mentioned photosensitive polyimide resin precursor composition to the pattern formation by the wet process to prepare the optical waveguide provided with a core layer comprising a polyimide resin, the pattern formation is conducted using the photosensitive polyimide resin precursor composition as a starting material, to which photosensitivity is imparted by blending a photosensitive agent with a polyamic acid which is a polyimide resin precursor, followed by heating to allow the polyamic acid to undergo ring closure and curing (imidation), as described above. In this case, there is the problem that the above-mentioned photosensitive agent is thermally decomposed to color the resulting polyimide resin black. Therefore, according to the wet process using the conventional photosensitive polyimide resin precursor composition as described above, improvement in the low loss property has a limitation even when the amount of the photosensitive agent used is reduced. It is therefore impossible to sufficiently exhibit the low loss property originally possessed by polyimide.
Patent Document 1: JP 2002-031732 A
Patent Document 2: JP 6-43648 A
Patent Document 3: JP 7-179604 A
Patent Document 4: JP 7-234525 A